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Inverters and AC motors

Discussion in 'Electronic Design' started by hamilton, Nov 10, 2012.

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  1. hamilton

    hamilton Guest

    With the current discussion of inverters and an electric blankeypoo,

    I would like to create a backup for the Gas-Forced-Air furnace in my house.

    The last time the power went out, the gas stove still had gas flowing.

    But with the power out, the furnace would not work.

    So, will a Pure Sine Wave inverter be able to run the motor in a
    standard furnace ?

    Thanks

    hamilton
     
  2. It should.

    furnace blowers are just plain capacitor run induction motors, with no
    crazy starting coils, so there's not a need for a hugely oversized
    inverter.
     
  3. P E Schoen

    P E Schoen Guest

    "hamilton" wrote in message
    You might not even need a pure sine wave. I was able to run a 1/2 to 3/4 HP
    pump motor on a 2000W Harbor Freight "modified sine wave" inverter that cost
    only about $100. I was also able to run a 2 HP three phase motor using a
    400W inverter and a voltage doubler to provide DC bus voltage for a VFD
    (which I had purchased new for about $60). And recently I bought a 1000W 220
    VAC inverter for about $40 and hooked it directly to two of the AC inputs of
    the VFD and the motor runs just fine, although I'm not sure how much load it
    will take. I might hack the inverter and access the internal DC bus which is
    probably about 300 VDC and perfect for the VFD's DC link.

    You might even consider a bank of 20-30 smaller 12V batteries in series to
    get the DC link voltage directly. I bought some 12V 12Ah SLAs on eBay for a
    little over $22 each, so for about $500 you can have 3.6 kWh capacity which
    should run a 1/4 HP (200W) furnace fan for about 12-15 hours total. Of
    course, you can also get 12V 100Ah flooded lead batteries for less than $100
    each.

    http://www.ebay.com/itm/320800175307 (1000W 220V inverter)
    http://www.ebay.com/itm/190701779364 (12V 12Ah batteries)
    http://www.ebay.com/itm/190750912593 (small VFD $50)
    http://www.ebay.com/itm/180906231025 (110V 2.2 kW single phase VFD)

    Paul
     
  4. gregz

    gregz Guest

    I don't know how much power mine takes, but it must have 16 wires going to
    the variable speed motor. Also makes a lot of RFI. I don't see me using an
    inverter but who knows.

    Just got to hooking up my solar panels.

    Greg
     
  5. furnaces have overly complex micro controlled nonsense in them these
    days, so not blowing out the electronics might be a consideration, over
    the older furnaces with a relay, gas valve and thermocouple wired to a
    thermostat.

    OP has a complex sounding motor too- are they no longer using one or two
    speed fan motors?
     
  6. hamilton

    hamilton Guest

    My thoughts on switch over is very very simple, manual switch over.

    Re-wire the furnace romex with a three prong plug/outlet.

    Plug the furnace into the three prong outlet.

    When the power goes out, un-plug the furnace for Public Service and plug
    into the inverter.
    I will look at Odyssey.

    Thanks

    hamilton
     
  7. Sylvia Else

    Sylvia Else Guest

    I once tried running a simple house fan on a modified sine way. Apart
    from a buzzing noise, it seemed OK, until the smell indicated that it
    was overheating. Presumably it didn't like the higher harmonic currents
    that were flowing.

    The message here being that the first indication you get that running
    with a modified sine way isn't a good idea may be that the motor stops -
    permanently.

    Sylvia.
     
  8. hamilton

    hamilton Guest

    Thank You Sylvia,

    This is the kind of information I am looking for.

    hamilton
     
  9. josephkk

    josephkk Guest

    Just the same it has to be motor rated. It has to be robust enough to
    handle a couple of seconds of 10X rated current. Cap run does not fix
    that.

    ?-)
     
  10. josephkk

    josephkk Guest

    Sounds like a poorly implemented variable frequency drive.

    ?-)
     
  11. gregz

    gregz Guest

    I just checked the current on my furnace with variable speed motor. The
    motor uses a whole bunch of wires with switching control. Not much current
    for exhaust and ignitors. Ramps slowly up to just under 3 amps 120 vac. Im
    not sure if it increases a bit if it goes into full btu mode. It's also a
    dual 50k 70k btu burner.
    Yes, I'm hooking my solar system, 6 panel harbor freight and just got a 1kw
    sine inverter. I have not hooked everything up, but I was impressed on 3
    panels delivering current on a cloudy day. I'll be taking more
    measurements. I didn't intend on using the inverter on furnace, but looks
    promising. Still got 3 gas generators. 1kw, 3.5 kw, 5kw .

    Greg
     
  12. josephkk

    josephkk Guest

    Maybe you have a high grade furnace with a VFD. How old is it? If it is
    more than 10 years old VFD is very unlikely. Two or three speed can be
    done with multiple windings with different pole counts (ceiling fans).
     
  13. they're just boring fan motors, no starting coils and they're not even all
    that powerful, so the inrush current won't be all that exciting. If I get
    bored this weekend I'll take some current readings off one (entire blower
    assembly from a furnace.)
     
  14. gregz

    gregz Guest

    It's 6 years old. Goodman.

    Greg
     
  15. Les Cargill

    Les Cargill Guest


    I've known a couple of people who did this:
    http://www.propane-generators.com/type_3.htm

    One was basically enough to run the 'fridge, freezer, and a few lights.
    It was an otherwise stock gasoline generator. In both cases,
    an inspection was done by a licensed plumber ( always a great idea when
    messing with natural gas ).
     
  16. tm

    tm Guest

    Iff you can recover the waste heat from a natural gas powered generator and
    use that for space heating, the efficiency gets crazy good. Trick would be
    to level out the electrical demand with some form of battery storage and
    conversion.
     
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